Steerable beam helix antenna are used for mobile communications, including land, sea and air-borne terminals, satellite communications and/or in situations where inference sources are suppressed by manipulation of the shape of the antenna to vary its radiation pattern.
The helix antenna is well-known, as are its many modes of operation; see for example the book "antennas" by John De Kraus 2nd edition, McGraw Hill Book Company, 1950 (herein referred to as "Kraus") at page 273. The particular mode of operation depends upon parameters of the antenna, such as the number of helical conductors, the pitch angle of the conductors, antenna length and, to a lesser extent, the conductor size. One mode of operation is the so called "scanning mode" of operation. In this mode the helix is of electrically small diameter (about 0.1 wavelengths), of large pitch angle (about 60 degrees) and has several turns. This mode is referred to as the "scanning mode" as for a fixed antenna construction the beam of the antenna can scan by varying the frequency of operation. This technique may be used in radar and direction finding equipment.
Alternatively, when operating at a nominally constant frequency, the radiation pattern of the antenna can be scanned by altering parameters of the antenna.
U.S. Pat. Nos. 3,524,193; 3,510,872; 4,475,111; 3,699,585; 3,836,979 and 4,068,238 disclose helical antennas of adjustable length. However, these antennas are foldable, collapsible or telescoping only for the purpose of enabling transportation in a more compact state.
U.S. Pat. No. 4,087,820 discloses a short wave antenna which allows the height of the antenna to be varied whilst the antenna pitch remains constant. This allows the antenna to be turned to resonance within a wide frequency range. The antenna is however extremely high and the construction would be unsuitable for antenna having a large pitch angle, especially when used in mobile applications.
U.S. Pat. No. 5,146,235 discloses a UHF antenna in which the number of turns or the height of the antenna may be adjusted. The conductor is a helical spring and its height and number of turns may be adjusted by mechanical means. Such a structure would be unsuitable for an antenna having a large pitch angle. The spring would tend to resonate, which would produce distortion, especially in multifilar antennas. The problem would be particularly apparent when the antenna was mounted to a moveable vehicle.